4,490 research outputs found
Fragment Formation in Central Heavy Ion Collisions at Relativistic Energies
We perform a systematic study of the fragmentation path of excited nuclear
matter in central heavy ion collisions at the intermediate energy of . The theoretical calculations are based on a Relativistic
Boltzmann-Uehling-Uhlenbeck () transport equation including stochastic
effects. A Relativistic Mean Field () approach is used, based on a
non-linear Lagrangian, with coupling constants tuned to reproduce the high
density results of calculations with correlations.
At variance with the case at Fermi energies, a new fast clusterization
mechanism is revealed in the early compression stage of the reaction dynamics.
Fragments appear directly produced from phase-space fluctuations due to
two-body correlations. In-medium effects of the elastic nucleon-nucleon cross
sections on the fragmentation dynamics are particularly discussed. The
subsequent evolution of the primordial clusters is treated using a simple
phenomenological phase space coalescence algorithm.
The reliability of the approach, formation and recognition, is investigated
in detail by comparing fragment momentum space distributions {\it and
simultaneously} their yields with recent experimental data of the
collaboration by varying the system size of the colliding system, i.e. its
compressional energy (pressure, radial flow). We find an excellent agreement
between theory and experiment in almost all the cases and, on the other hand,
some limitations of the simple coalescence model. Furthermore, the temporal
evolution of the fragment structure is explored with a clear evidence of an
earlier formation of the heavier clusters, that will appear as interesting
of the high density phase of the nuclear Equation of State ().Comment: 21 pages, 8 figures, Latex Elsart Style, minor corrections in p.7,
two refs. added, Nucl.Phys.A, accepte
Monte-Carlo calculation of longitudinal and transverse resistivities in a model Type-II superconductor
We study the effect of a transport current on the vortex-line lattice in
isotropic type-II superconductors in the presence of strong thermal
fluctuations by means of 'driven-diffusion' Monte Carlo simulations of a
discretized London theory with finite magnetic penetration depth. We calculate
the current-voltage (I-V) characteristics for various temperatures, for
transverse as well as longitudinal currents I. From these characteristics, we
estimate the linear resistivities R_xx=R_yy and R_zz and compare these with
equilibrium results for the vortex-lattice structure factor and the helicity
moduli. From this comparison a consistent picture arises, in which the melting
of the flux-line lattice occurs in two stages for the system size considered.
In the first stage of the melting, at a temperature T_m, the structure factor
drops to zero and R_xx becomes finite. For a higher temperature T_z, the second
stage takes place, in which the longitudinal superconducting coherence is lost,
and R_zz becomes finite as well. We compare our results with related recent
numerical work and experiments on cuprate superconductors.Comment: 4 pages, with eps figure
Analysis of Basis Pursuit Via Capacity Sets
Finding the sparsest solution for an under-determined linear system
of equations is of interest in many applications. This problem is
known to be NP-hard. Recent work studied conditions on the support size of
that allow its recovery using L1-minimization, via the Basis Pursuit
algorithm. These conditions are often relying on a scalar property of
called the mutual-coherence. In this work we introduce an alternative set of
features of an arbitrarily given , called the "capacity sets". We show how
those could be used to analyze the performance of the basis pursuit, leading to
improved bounds and predictions of performance. Both theoretical and numerical
methods are presented, all using the capacity values, and shown to lead to
improved assessments of the basis pursuit success in finding the sparest
solution of
Dynamics in Colloidal Liquids near a Crossing of Glass- and Gel-Transition Lines
Within the mode-coupling theory for ideal glass-transitions, the mean-squared
displacement and the correlation function for density fluctuations are
evaluated for a colloidal liquid of particles interacting with a square-well
potential for states near the crossing of the line for transitions to a gel
with the line for transitions to a glass. It is demonstrated how the dynamics
is ruled by the interplay of the mechanisms of arrest due to hard-core
repulsion and due to attraction-induced bond formation as well as by a nearby
higher-order glass-transition singularity. Application of the universal
relaxation laws for the slow dynamics near glass-transition singularities
explains the qualitative features of the calculated time dependence of the
mean-squared displacement, which are in accord with the findings obtained in
molecular-dynamics simulation studies by Zaccarelli et. al [Phys. Rev. E 66,
041402 (2002)]. Correlation functions found by photon-correlation spectroscopy
in a micellar system by Mallamace et. al [Phys. Rev. Lett. 84, 5431 2000)] can
be interpreted qualitatively as a crossover from gel to glass dynamics.Comment: 13 pages, 12 figure
Self-organizing & stochastic behaviors during the regeneration of hair stem cells
Stem cells cycle through active and quiescent states. Large populations of stem cells in an organ may cycle randomly or in a coordinated manner. Although stem cell cycling within single hair follicles has been studied, less is known about regenerative behavior in a hair follicle population. By combining predictive mathematical modeling with in vivo studies in mice and rabbits, we show that a follicle progresses through cycling stages by continuous integration of inputs from intrinsic follicular and extrinsic environmental signals based on universal patterning principles. Signaling from the WNT/bone morphogenetic protein activator/inhibitor pair is coopted to mediate interactions among follicles in the population. This regenerative strategy is robust and versatile because relative activator/inhibitor strengths can be modulated easily, adapting the organism to different physiological and evolutionary needs
First-Order Melting of a Moving Vortex Lattice: Effects of Disorder
We study the melting of a moving vortex lattice through numerical simulations
with the current driven 3D XY model with disorder. We find that there is a
first-order phase transition even for large disorder when the corresponding
equilibrium transition is continuous. The low temperature phase is an
anisotropic moving glass.Comment: Important changes from original version. Finite size analysis of
results has been added. Figure 2 has been changed. There is a new additional
Figure. To be published in Physical Review Letter
Geometric Tachyon to Universal Open String Tachyon
A system of k Neveu-Schwarz (NS) 5-branes of type II string theory with one
transverse direction compactified on a circle admits various unstable D-brane
systems, - some with geometric instability arising out of being placed at a
point of unstable equilibrium in space and some with the usual open string
tachyonic instability but no geometric instability. We discuss the effect of NS
5-branes on the descent relations among these branes and their physical
interpretation in the T-dual ALF spaces. We argue that if the tachyon potential
controlling these descent relations obeys certain conditions, then in certain
region in the parameter space labelling the background the two types of
unstable branes become identical via a second order phase transition, with the
geometric tachyon in one system getting mapped to the open string tachyon of
the other system. This would provide a geometric description of the tachyonic
instability of the usual non-BPS Dp-brane in ten dimensional flat space-time.Comment: LaTeX file, 30 page
Evidence for a Two-stage Melting Transition of the Vortex Matter in Bi2Sr2Ca1Cu2O8+d Single Crystals obtained by Muon Spin Rotation
From muon spin rotation measurements on under- to overdoped Bi-2212 crystals
we obtain evidence for a two-stage transition of the vortex matter as a
function of temperature. The first transition is well known and related to the
irreversibility line (IL). The second one is located below the IL and has not
been previously observed. It occurs for all three sets of crystals and is
unrelated to the vortex mobility. Our data are consistent with a two-stage
melting scenario where the intra-planar melting of the vortex lattice and the
inter-planar decoupling of the vortex lines occur independently.Comment: 9 pages and 3 figure
Entanglement of electrons in interacting molecules
Quantum entanglement is a concept commonly used with reference to the
existence of certain correlations in quantum systems that have no classical
interpretation. It is a useful resource to enhance the mutual information of
memory channels or to accelerate some quantum processes as, for example, the
factorization in Shor's Algorithm. Moreover, entanglement is a physical
observable directly measured by the von Neumann entropy of the system. We have
used this concept in order to give a physical meaning to the electron
correlation energy in systems of interacting electrons. The electronic
correlation is not directly observable, since it is defined as the difference
between the exact ground state energy of the many--electrons Schroedinger
equation and the Hartree--Fock energy. We have calculated the correlation
energy and compared with the entanglement, as functions of the nucleus--nucleus
separation using, for the hydrogen molecule, the Configuration Interaction
method. Then, in the same spirit, we have analyzed a dimer of ethylene, which
represents the simplest organic conjugate system, changing the relative
orientation and distance of the molecules, in order to obtain the configuration
corresponding to maximum entanglement.Comment: 15 pages, 7 figures, standard late
Composite fluxbranes with general intersections
Generalized composite fluxbrane solutions for a wide class of intersection
rules are obtained. The solutions are defined on a manifold which contains a
product of n Ricci-flat spaces M_1 x ... x M_n with 1-dimensional M_1. They are
defined up to a set of functions H_s obeying non-linear differential equations
equivalent to Toda-type equations with certain boundary conditions imposed. A
conjecture on polynomial structure of governing functions H_s for intersections
related to semisimple Lie algebras is suggested. This conjecture is valid for
Lie algebras: A_m, C_{m+1}, m > 0. For simple Lie algebras the powers of
polynomials coincide with the components of the dual Weyl vector in the basis
of simple roots. Explicit formulas for A_1 + ... + A_1 (orthogonal),
"block-ortogonal" and A_2 solutions are obtained. Certain examples of solutions
in D = 11 and D =10 (II A) supergravities (e.g. with A_2 intersection rules)
and Kaluza-Klein dyonic A_2 flux tube, are considered.Comment: 19 pages, Latex, 1 reference (on a pioneering paper of Gibbons and
Wiltshire) and two missing relations are added Published: Class. Quantum
Grav. 19 (2002) 3033-304
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